Abstract
A novel application of velocity-map imaging (VMI) is demonstrated, whereby the momentum distribution of photoelectrons ejected from a tungsten nanoscale metal tip (< 50 nm radius) is recorded following illumination with an ultrafast laser pulse. The electrostatic conditions in the VMI instrument are optimized through finite element modelling, taking into account a physically realistic geometry including all conductive elements in the vicinity of the electron trajectories. The instrument is calibrated by observing above threshold ionization in krypton gas, and simultaneous electron emission from this gas and a tungsten nanotip is presented, illustrating that the velocity mapping condition is maintained. Realizing photoelectron VMI for femtosecond laser illuminated nanoscale objects will have a significant impact on the emerging field of ultrafast nanoplasmonics and will influence the development of such devices as a source of coherent pulses of electrons with applications in time-resolved microscopy, holography and diffractive imaging.
Highlights
Focusing an ultrafast laser pulse into an effusive or supersonic gas target generates a sufficiently strong electric field to initiate ionization and fragmentation
The dynamics of electronic wavepackets can be directly driven by manipulating the carrier-envelope phase (CEP) of the laser pulse if it is of a few optical cycles in duration [4, 5]
We look to demonstrate Velocity map imaging (VMI) of photoelectron emission from a metal nanotip illuminated with a strong field laser pulse, an area of active research worldwide [12, 13]
Summary
Focusing an ultrafast laser pulse into an effusive or supersonic gas target generates a sufficiently strong electric field to initiate ionization and fragmentation. It is relatively straightforward to have this condition apply over a large volume: a VMI apparatus of dimensions hundreds of millimetres will successfully momentum image over of the order of millimetres to tens of millimetres This is of particular relevance to strong-field ultrafast laser dynamics, whereby a laser pulse is focused down to a beam waist on the order of microns, causing nonlinear processes such as tunnel ionization, recollision excitation and ionization, formation of rotational and vibrational wavepackets and molecular fragmentation. The radius of curvature at the tip of such objects (typically tens of nanometres) enhances the electric field induced by the incident radiation field [15,16,17], electron tunnelling into the continuum can be caused by very low fluences as compared to a gas phase atomic or molecular target [18,19,20,21,22] Such devices are attracting significant interest at the interface between ultrafast atomic physics, nanoscale plasmonics and electron microscopy. An accurate understanding of the angular spread, absolute momentum and bandwidth of the electron pulses is vital for their application to time-resolved imaging via diffraction and microscopy [34]
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